Cytochrome c oxidase is the terminal enzyme of cellular respiration in all higher aerobic organisms. It functions to catalyze the reduction of dioxygen to water, coupling the resultant energy through the cytochrome chain into the oxidative phosphorilation process. There is no enzyme system in any higher animal that is more important than cytochrome oxidase. Its metabolic role is essential to life. In spite of being one of the most studied enzyme systems in existence, it also remains one of the most enigmatic. By studying well chosen model compounds we can increase our understanding of this most important enzyme. The goals of this investigation are: a) To prepare Fe/Cu complexes which mimic the active site in cytochrome c oxidase, b) To examine the physical and chemical properties of these complexes and c) To examine the mechanism of their reaction with dioxygen. The types of complexes proposed are derived from meso-alpha, alpha, alpha, alpha,-substituted tetraphenylporphyrins. A second tetradentate coordination site will be "built" into the molecule by appending four additional ligands to the porphyrin through reactions of each of the four ortho substituents. Iron/copper mixed-metal complexes of these ligands will be prepared and studied by the same series of methods and techniques that have been employed to study the natural enzyme. These will include: EPR, electrochemistry, magnetic susceptibility, optical spectroscopy, Mossbauer spectroscopy, ligand binding studies and simple reaction rate studies. The results of the model studies will be used in an effort to try to better understand similar data which exists for the enzyme.